refactor: adds schemales changeset and validation constant

docs/email-validation.md

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+# Email Validation Strategy
+
+We use `EctoCommons.EmailValidator` with both `:html_input` and `:pow` checks, defined centrally in `Mv.Constants.email_validator_checks/0`.
+
+## Checks Used
+
+- `:html_input` - Pragmatic validation matching browser `<input type="email">` behavior
+- `:pow` - Stricter validation following email spec, supports internationalization (Unicode)
+
+## Rationale
+
+Using both checks ensures:
+- **Compatibility with common email providers** (`:html_input`) - Matches what users expect from web forms
+- **Compliance with email standards** (`:pow`) - Follows RFC 5322 and related specifications
+- **Support for international email addresses** (`:pow`) - Allows Unicode characters in email addresses
+
+This dual approach provides a balance between user experience (accepting common email formats) and technical correctness (validating against email standards).
+
+## Usage
+
+The checks are used consistently across all email validation points:
+
+- `Mv.Membership.Import.MemberCSV.validate_row/3` - CSV import validation
+- `Mv.Membership.Member` validations - Member resource validation
+- `Mv.Accounts.User` validations - User resource validation
+
+All three locations use `Mv.Constants.email_validator_checks()` to ensure consistency.
+
+## Implementation Details
+
+### CSV Import Validation
+
+The CSV import uses a schemaless changeset for email validation:
+
+```elixir
+changeset =
+  {%{}, %{email: :string}}
+  |> Ecto.Changeset.cast(%{email: Map.get(member_attrs, :email)}, [:email])
+  |> Ecto.Changeset.update_change(:email, &String.trim/1)
+  |> Ecto.Changeset.validate_required([:email])
+  |> EctoCommons.EmailValidator.validate_email(:email, checks: Mv.Constants.email_validator_checks())
+```
+
+This approach:
+- Trims whitespace before validation
+- Validates email is required
+- Validates email format using the centralized checks
+- Provides consistent error messages via Gettext
+
+### Resource Validations
+
+Both `Member` and `User` resources use similar schemaless changesets within their Ash validations, ensuring consistent validation behavior across the application.
+
+## Changing the Validation Strategy
+
+To change the email validation checks, update the `@email_validator_checks` constant in `Mv.Constants`. This will automatically apply to all validation points.
+
+**Note:** Changing the validation strategy may affect existing data. Consider:
+- Whether existing emails will still be valid
+- Migration strategy for invalid emails
+- User communication if validation becomes stricter
+